Sex differences in sensory and motor branches of the pudendal nerve of the rat

Sexually selected size differences and conserved sexual monomorphism of genital cortex

The mammalian somatosensory cortex shows marked species‐specific differences. How evolution in general and sexual selection in particular shape the somatosensory cortical body representation has not been delineated, however. Here we address this issue by a comparative analysis of genital cortex. Genitals are unique body parts in that they show sexual dimorphism, major changes in puberty and typically more pronounced species differences than other body parts (Hosken & Stockley, 2004). To study the evolution of genital cortex we flattened cortical hemispheres and assembled 104 complete body maps, revealed by cytochrome‐oxidase activity in layer 4 of 8 rodent and 1 lagomorph species. In two species, we also performed antibody stainings against vesicular glutamate transporter‐2, which suggested that cytochrome‐oxidase maps closely mirror thalamic innervation. We consistently observed a protrusion between hindlimb and forelimb representation, which in rats (Lenschow et al., 2016) corresponds to the penis representation in males and the clitoris representation in females. Consistent with the idea that this protrusion corresponds to genital cortex, we observed a size increase of this protrusion during puberty. Species differed in external genital sexual dimorphism, but we observed a sexual monomorphism of the putative genital protrusion in all species, similar to previous observations in rats. The relative size of the putative genital protrusion varied more than 3‐fold between species ranging from 0.5% of somatosensory cortex area in chipmunks to 1.7% in rats. This relative size of the genital protrusion co‐varied with relative testicle size, an indicator of sperm competition and sexual selection.

Anatomical organization and somatic axonal components of the lumbosacral nerves in female rabbits

AIM

To determine the anatomical organization and somatic axonal components of the lumbosacral nerves in female rabbits.

METHODS

Chinchilla adult anesthetized female rabbits were used. Anatomical, electrophysiological, and histological studies were performed.

RESULTS

L7, S1, and some fibers from S2 and S3 form the lumbosacral trunk, which gives origin to the sciatic nerve and innervation to the gluteal region. From S2 to S3 originates the pudendal nerve, whose branches innervates the striated anal and urethra sphincters, as well as the bulbospongiosus, ischiocavernosus, and constrictor vulvae muscles. The sensory field of the pudendal nerve is ∼1800 mm² and is localized in the clitoral sheath and perineal and perigenital skin. The organization of the pudendal nerve varies between individuals, three patterns were identified, and one of them was present in 50% of the animals. From S3 emerge the pelvic nerve, which anastomoses to form a plexus localized between the vagina and the rectum. The innervation of the pelvic floor originates from S3 to S4 fibers.

CONCLUSIONS

Most of the sacral spinal nerves of rabbit are mixed, carrying sensory, and motor information. Sacral nerves innervate the hind limbs, pelvic viscera, clitoris, perineal muscles, inguinal and anal glands and perineal, perigenital, and rump skin. The detailed description of the sacral nerves organization, topography, and axonal components further the knowledge of the innervation in pelvic and perinal structures of the female rabbit. This information will be useful in future studies about the physiology and physiopathology of urinary, fecal, reproductive, and sexual functions.

Sexually Monomorphic Maps and Dimorphic Responses in Rat Genital Cortex

Mammalian external genitals show sexual dimorphism [1, 2] and can change size and shape upon sexual arousal. Genitals feature prominently in the oldest pieces of figural art [3] and phallic depictions of penises informed psychoanalytic thought about sexuality [4, 5]. Despite this longstanding interest, the neural representations of genitals are still poorly understood [6]. In somatosensory cortex specifically, many studies did not detect any cortical representation of genitals [7-9]. Studies in humans debate whether genitals are represented displaced below the foot of the cortical body map [10-12] or whether they are represented somatotopically [13-15]. We wondered what a high-resolution mapping of genital representations might tell us about the sexual differentiation of the mammalian brain. We identified genital responses in rat somatosensory cortex in a region previously assigned as arm/leg cortex. Genital responses were more common in males than in females. Despite such response dimorphism, we observed a stunning anatomical monomorphism of cortical penis and clitoris input maps revealed by cytochrome-oxidase-staining of cortical layer 4. Genital representations were somatotopic and bilaterally symmetric, and their relative size increased markedly during puberty. Size, shape, and erect posture give the cortical penis representation a phallic appearance pointing to a role in sexually aroused states. Cortical genital neurons showed unusual multi-body-part responses and sexually dimorphic receptive fields. Specifically, genital neurons were co-activated by distant body regions, which are touched during mounting in the respective sex. Genital maps indicate a deep homology of penis and clitoris representations in line with a fundamentally bi-sexual layout [16] of the vertebrate brain.

An afferent explanation for sexual dimorphism in the aortic baroreflex of rat

Sex differences in baroreflex (BRx) function are well documented. Hormones likely contribute to this dimorphism, but many functional aspects remain unresolved. Our lab has been investigating a subset of vagal sensory neurons that constitute nearly 50% of the total population of myelinated aortic baroreceptors (BR) in female rats but less than 2% in male rats. Termed "Ah," this unique phenotype has many of the nonoverlapping electrophysiological properties and chemical sensitivities of both myelinated A-type and unmyelinated C-type BR afferents. In this study, we utilize three distinct experimental protocols to determine if Ah-type barosensory afferents underlie, at least in part, the sex-related differences in BRx function. Electron microscopy of the aortic depressor nerve (ADN) revealed that female rats have less myelin (P < 0.03) and a smaller fiber cross-sectional area (P < 0.05) per BR fiber than male rats. Electrical stimulation of the ADN evoked compound action potentials and nerve conduction profiles that were markedly different (P < 0.01, n = 7 females and n = 9 males). Selective activation of ADN myelinated fibers evoked a BRx-mediated depressor response that was 3-7 times greater in female (n = 16) than in male (n = 17) rats. Interestingly, the most striking hemodynamic difference was functionally dependent upon the rate of myelinated barosensory fiber activation. Only 5-10 Hz of stimulation evoked a rapid, 20- to 30-mmHg reduction in arterial pressure of female rats, whereas rates of 50 Hz or higher were required to elicit a comparable depressor response from male rats. Collectively, our experimental results are suggestive of an alternative myelinated baroreceptor afferent pathway in females that may account for, at least in part, the noted sex-related differences in autonomic control of cardiovascular function.

Genitourinary dysfunction in male rats after bilateral neurectomy of the motor branch of the sacral plexus

AIMS

To determine the contribution of the striated musculature anatomically related to the urethra on urinary continence in conscious male rats. We tested whether the bilateral neurectomy of the motor branch of the sacral plexus (MBSP), a nerve that innervates the bulbospongiosus, ischiocavernosus, and external urethral sphincter, is a reliable procedure to induce changes in voiding pattern that can be used as indicators of urinary incontinence in unanesthetized male rats.

METHODS

Micturition behavior was videotaped and urinary parameters measured 24 h before and at day 2 and 10 after surgery.

RESULTS

Intact animals have a stereotyped behavior of micturition consisting in urination in the corner of the cage. Neurectomized animals lost place preference for voiding demonstrated by leakage of urine throughout the cage while eating, walking, or sleeping. Voiding frequency was double and voiding duration was triple the amount before surgery. Urine flow rate and voiding volume were also significantly decreased. Necropsy showed that 10 days post-denervation semen material was accumulated in the urethra and in the bladder.

CONCLUSION

In male rats the perineal striated muscles are crucial to maintaining normal urinary continence, preventing retrograde ejaculation, and to expelling urine and seminal secretions. Bilateral neurectomy of the MBSP may not be appropriate for long term survival urinary studies because effects on urinary parameters can be contaminated and/or masked by impaired seminal fluid expulsion, as a consequence of impairment of striated urethral muscle function.

Effects of high-frequency alternating current on axonal conduction through the vagus nerve

High-frequency alternating current (HFAC) is known to disrupt axonal conduction in peripheral nerves, and HFAC has much potential as a therapeutic approach for a number of pathological conditions. Many previous studies have utilized motor output as a bioassay of effects of HFAC on conduction through medium- to large-diameter motor axons. However, little is known about the effectiveness of HFAC on smaller, more slowly conducting nerve fibres. The present study tested whether HFAC influences axonal conduction through sub-diaphragmatic levels of the rat vagus nerve, which consists almost entirely of small calibre axons. Using an isolated nerve preparation, we tested the effects of HFAC on electrically evoked compound action potentials (CAPs). We found that delivery of charge-balanced HFAC at 5000 Hz for 1 min was effective in producing reversible blockade of axonal conduction. Both Aδ and C components of the vagus CAP were attenuated, and the degree of blockade as well as time to recovery was proportional to the amount of HFAC current delivered. The Aδ waves were more sensitive than C waves to HFAC blockade, but they required more time to recover.

Maternal care effects on SNB motoneuron development: the mediating role of sensory afferent distribution and activity

Maternal licking in rats affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Reduced maternal licking produces decreased motoneuron number, size, and dendritic length in the rostral portion of the adult SNB as well as deficits in adult male copulatory behavior. Previous research suggests that decreases in perineal tactile stimulation may be responsible for these effects. To determine whether the regional effects of maternal licking on SNB morphology are driven by sensory afferent innervation of the lumbosacral spinal cord, we used WGA-HRP to reconstruct the location of sensory afferent fibers from the perineal skin. We found that these fibers are caudally concentrated relative to the area of the SNB dendritic field, with the rostral dendritic arbor receiving little perineal afferent innervation. We also assessed Fos expression following perineal tactile stimulation to determine whether it increased local spinal cord activity in the SNB dendritic field. Sixty seconds of licking-like perineal stimulation produced a transient 115% increase in Fos expression in the area of the SNB dendritic field. This effect was driven by a significant increase in Fos in the caudal portion of the SNB dendritic field, matching the pattern of perineal afferent fiber labeling. Perineal tactile stimulation also produced significantly greater Fos expression in male pups than in female pups. Together, these results suggest that perineal sensory afferent activity mediates the effects of early maternal care on the masculinization of the SNB and resultant male copulatory behavior.

Neuropeptide modulation of a lumbar spinal reflex: potential implications for female sexual function

INTRODUCTION

Neuropeptides are known to modulate female receptivity. However, even though receptivity is a spinal reflex, the role of neuropeptides within the spinal cord remains to be elucidated.

AIM

The aims were to (i) investigate neuropeptides in the lumbosacral region; and (ii) determine how neuropeptides modulate glutamate release from stretch Ia fibers, touch sensation Abeta fibers and Adelta/C pain fibers.

MAIN OUTCOME MEASURES

Neuropeptide modulation of the lumbosacral dorsal-root ventral-root reflex in vitro.

METHODS

Spinal cords were removed from Sprague-Dawley rats in compliance with UK Home Office guidelines. Hemisected cords were superfused with aCSF and the dorsal root (L4-S1) was stimulated to evoke glutamate release. A biphasic reflex response was evoked from the opposite ventral root consisting of a monosynaptic (Ia fibers) and polysynaptic (Abeta, Adelta/C fibers) component.

RESULTS

The micro opioid receptor (MOR) agonist DAMGO inhibited the monosynaptic (EC(50) 0.02 +/- 0.02 nM) and polysynaptic area (EC(50) 125 +/- 167 nM) but not polysynaptic amplitude. Oxytocin and corticotrophin releasing factor (CRF) inhibited the monosynaptic amplitude (EC(50), 1.4 +/- 1.0 nM and EC(50) 4.3 +/- 3.5 nM, respectively), polysynaptic amplitude (EC(50) 18.2 +/- 28.0 nM and EC(50), 9.5 +/- 13.3 nM, respectively), and area (EC(50) 11.6 +/- 13.0 nM and EC(50), 2.8 +/- 3.3 nM, respectively); effects that were abolished by oxytocin and CRF(1) antagonists, L-368899 and 8w. Melanocortin agonists solely inhibited the monosynaptic component, which were blocked by the MC(3/4) receptor antagonist SHU9119.

CONCLUSION

These data suggest endogenous neuropeptides are released within the lumbosacral spinal cord. Melanocortin agonists, oxytocin, CRF, and DAMGO via MC(4), oxytocin, CRF(1), and MOR inhibit glutamate release but with differing effects on afferent fiber subtypes. Melanocortins, oxytocin, CRF, and DAMGO have the ability to modulate orgasm whereas oxytocin, CRF and DAMGO can increase pain threshold. Oxytocin and CRF may dampen touch sensation.

Neuroactive Steroid Levels in a transgenic rat model of CMT1A Neuropathy

Charcot-Marie-Tooth type 1A (CMT1A) represents 80% of all the demyelinating hereditary motor and sensory neuropathies. As recently suggested, neuroactive steroids may have a role in a therapeutic strategy for peripheral neuropathies, including CMT1A. To this aim, an accurate qualitative and quantitative analysis of neuroactive steroid levels in this disease could be extremely important to define effective pharmacological strategies. We here analyzed by liquid chromatography-tandem mass spectrometry the levels of neuroactive steroids present in the sciatic nerve of male and female peripheral myelin protein 22 transgenic rats (PMP22(tg) rats; i.e., an experimental model of CMT1A) and of the corresponding wild-type littermates. We observed that, both in PMP22(tg) rats and in the wild types, the levels of neuroactive steroids, such as progesterone, tetrahydroprogesterone (THP), isopregnanolone (3beta,5alpha-THP), testosterone, dihydrotestosterone, and 5alpha-androstane-3alpha, 17beta-diol (3alpha-diol) are sexually dimorphic. It is interesting to note that the levels of 3beta,5alpha-THP and of 3alpha-diol, which are exclusively detectable in sciatic nerve of female and male rats, respectively, are strongly decreased in PMP22(tg) rats. 3beta,5alpha-THP and 3alpha-diol are modulators of gamma-amino butyric acid A receptor. Thus, the present findings may be considered an interesting background for experiments aimed to evaluate the possible therapeutic effects of modulators of this neurotransmitter receptor in male and female PMP22(tg) rats.

Sex-dimorphic effects of progesterone and its reduced metabolites on gene expression of myelin proteins by rat Schwann cells

Data obtained in our and other laboratories have indicated that progesterone (P) and its derivatives, dihydroprogesterone (DHP) and tetrahydroprogesterone (THP), stimulate the expression of two myelin proteins of the peripheral nervous system (PNS) [i.e., glycoprotein zero (P0) and peripheral myelin protein 22 (PMP22)]. We have now considered the effects of P and its derivatives on these and other myelin proteins [i.e., myelin-associated glycoprotein (MAG) and myelin and lymphocyte protein (MAL)] in sex-specific cultures of rat Schwann cells. Gene expression of myelin proteins was assessed by RNase protection assay. Treatment with P or DHP induced a stimulatory effect on P0 mRNA levels in male but not in female Schwann cells. In contrast, treatment with THP increased gene expression of P0 exclusively in female Schwann cells. A similar sex-difference was also evident for other myelin proteins. Indeed, PMP22 expression was stimulated by treatment with P in male cultures, whereas THP induced an increase of mRNA levels in female cultures. Moreover, MAG was stimulated by THP treatment in male cultures only, whereas MAL expression was unaffected by neuroactive steroid treatment in both male and female cultures. In conclusion, the present observations indicate that the effects of neuroactive steroids on myelin proteins are sexually dimorphic. This finding might represent an important background for sex-specific therapies of acquired and inherited peripheral neuropathies.

Ascending spinal pathways from sexual organs: effects of chronic spinal lesions

A recent survey of paraplegics indicates that regaining sexual function is of the highest priority for both males and females (Anderson, K.D. (2004) Targeting recovery: priorities of the spinal cord-injured population J. Newrotrauma, 21: 1371-1383). Our understanding of the neural pathways and mechanisms underlying sexual behavior and function is limited at the present time. More studies are obviously needed to direct experiments geared toward developing effective therapeutic interventions. In this chapter, a review of studies on the processing of sensory inputs from the male and female reproductive organs is presented with a review of what is known about the location of ascending spinal pathways conveying this information. The effect of spinal cord injury on sexual function and the problems that ensue are discussed.

Maternal behavior, infant development, and the question of developmental resources

The natural development of maternal and infant behavior occurs in a dyad characterized by synchrony and reciprocal interactions. Major concepts used to describe and analyze this synchrony were reviewed. It was concluded that the dyad undergoes a developmental progression in which each part of the dyad is both a developing organism and a reliably changing milieu forming part of the extended inheritance of the other. The reliability of inherited resources is rooted in interactions essential to life, such as those used to transfer metabolic needs to dependent offspring; to stimulation (incidentally but necessarily) associated with life-supporting mechanisms; and to perceptual, motor, or learning mechanisms used to extract specific resources from the available milieu. The diverse resources in extended inheritance contribute to the construction of new traits through opportunistic shaping or regulating interactions among them that are unrestricted by their function at earlier stages.

Perinatal exposure to androgen suppresses sexual dimorphism in nerve trunk diameter, axon number, and fiber size spectrum: a quantitative ultrastructural study of the adult rat mammary nerve

The adult mammary nerves (MNs) from female, male, and testosterone-androgenized female rats were studied by light and electron microscopy. The female MNs trunk has twice the diameter of that of the male. Morphometry showed a significantly more myelinated (307 +/- 6) and unmyelinated axons (1654 +/- 10) in the female MN than the male MN (278 +/- 6 and 1373 +/- 28, respectively). Perinatal exposure of the female to testosterone significantly reduced the number of both axon types in the MN in adulthood (244 +/- 6 myelinated and 1300 +/- 32, unmyelinated). Another sexual dimorphism is a distinct group of large (>7.0 microm in diameter) myelinated axons known to conduct sensory information (i.e., touch and vibration). Because the male and the perinatally-androgenized female MNs lack these fibers, it is concluded that gonadal sex hormones may promote the differentiation of specific sets of axons committed to transmission of sensory cues relevant to reproduction.

Pattern of sensory innervation of the perineal skin in the female rat

Here we describe the nerves innervating the perineal skin together with their sensory fields in the adult female rat. Electrophysiological recording showed that the lumbosacral and L6-S1 trunks, in part by way of the sacral plexus, transmit sensory information from the perineal skin via four nerves: the viscerocutaneous branch of the pelvic nerve innervating the skin at the midline between the vaginal opening and anus, the sensory branch of the pudendal nerve innervating the clitoral sheath, the distal perineal branch of the pudendal nerve innervating a broad area of skin adjacent to the vaginal opening and anus, and the proximal perineal branch of the sacral plexus innervating a broad area of skin adjacent to the clitoris and vaginal opening. The sensory fields of three of these nerves overlapped to some degree: the viscerocutaneous branch of the pelvic and the distal perineal branch of the pudendal nerves at the midline skin between the vaginal opening and the anus, and the distal perineal branch of the pudendal nerve and the proximal perineal branch of the sacral plexus at the skin lateral to the vaginal opening. Such overlap might provide a safeguard helping to ensure that somatosensory input from the perineal region important for triggering reproductive and nonreproductive reflexes reaches the CNS.

Effects of neonatal testosterone treatment on pacing behaviors and development of a conditioned place preference

Two experiments assessed the effects of neonatal testosterone treatment on paced mating behavior and conditioned place preference in female rats. In both experiments, females received s.c. injections of 5.0 microg testosterone propionate or oil vehicle at three days postpartum. As adults, females were ovariectomized and given s.c. injections of 10 microg estradiol benzoate and 500 microg progesterone, 48 and 4 h before mating, respectively. In Experiment 1, TP- and Oil-treated females exhibited similar high levels of lordosis responsiveness, but TP-treated females showed increased intervals between mounts and between intromissions in paced and non-paced mating conditions compared to control females. The effect was particularly pronounced during paced mating, when contact return latencies were increased approximately 2-fold by TP treatment. TP-treated females showed exaggerated pacing behavior, showing significantly greater return latencies after intromissions than Oil-treated females. In Experiment 2, TP- and Oil-treated groups were tested in a conditioned place preference paradigm to determine if the behavioral changes observed in Experiment 1 were in part a result of changes in the perceived reward produced by paced mating. TP treated and control females developed equivalent preferences for places associated with paced but not non-paced mating, indicating that neonatal TP treatment at this dosage does not disrupt or enhance the conditioned place preference induced by paced mating. The results of the two experiments demonstrate that neonatal TP treatment alters the display of pacing behavior but not the reward state induced by paced mating, and suggest that TP affects neural substrates involved in performance of paced mating without effects on those controlling lordosis or place preference conditioning.

Motor pudendal nerve characterization in the female rat

The aim of our study was to provide quantitative data on pudendal motor neuron cell bodies and axons in the female rat. To confirm earlier studies, fluorescent retrograde tracers were used to label the motor neurons for correlation with myelinated axon counts along the length of the motor pudendal nerve. The external urethral sphincter of female rats was injected with diamidino yellow and the external anal sphincter with fast blue. The L(6) spinal cord revealed labeled motor neurons. Those in the dorsolateral column (60.8 +/- 10.6) had nuclei labeled yellow from the external urethral sphincter and those in the dorsomedial column (31.7 +/- 8.5) had cytoplasm labeled blue from the external anal sphincter. Double labeling was not present, suggesting that pudendal motor neurons in each column innervate separate sphincters. The motor pudendal nerve in the ischiorectal fossa was also characterized by light microscopy. The mean myelinated axon count (151.4 +/- 17.0) was highly correlated (r = 0.995) in the proximal fascicles and the sum of distal fascicles. This indicated that myelinated axons do not branch at the point where the main motor pudendal nerve branches into separate fascicles. Axon counts between sides were not as well correlated (r = 0.883). The ratio of motor neurons to myelinated axons is 56%, suggesting that some myelinated axons either innervate other muscles or are sensory. This reproducible characterization of the normal pudendal nerve anatomy provides an excellent basis for experimental studies associated with pudendal nerve denervation as a model for neurogenic incontinence.

Variation in reproductive traits is associated with short anogenital distance in female rats

Anogenital distance was used as a biomarker for natural variation in prenatal androgenization of female rats and was associated with individual differences in a suite of reproductive characteristics. Females with short anogenital distances were likely to have first vaginal estrus earlier than females with longer anogenital distances and to have first vaginal estrus on the same day as vaginal opening. In young adulthood, females with short anogenital distances had shorter ovarian cycles and less intense lordosis reflexes in response to manual palpation than those with longer anogenital distances, but only when living individually, not in groups. Taken together, these findings indicate that prenatal androgenization affects reproductive traits throughout the life span.